Olivine is a naturally-occurring mineral having the nominal formula (Mg,Fe).sub.2 SiO.sub.4. It is commonly olive-green in color, has a Mohs hardness of 6.5 to 7, a specific gravity of about 3.2-3.3 and is difficultly fusible.
Olivine, also known as chrysolite, is gaining favor as a foundry sand in preference to silicas such as quartz because of the advantages it offers over silica. Olivine avoids the silicosis hazard of ordinary foundry sands and allows higher casting temperatures. While not as refractory as chromite or zircon, olivine does allow casting at temperatures some 300.degree. to 400.degree. F. higher than is possible using silica sands.
Sources of olivine include the igneous rocks such as peridotite, norite, basalt, diabase and gabbro and as the product of metamorphism of certain sedimentary rocks containing magnesia and silica such as impure dolomites. Olivine may be present as an accessory component or may make up the main rock constituent as in the dunite variety of peridotite. Olivine foundry sand is presently produced at two locations in this country. One is in the State of Washington and the other is in the western part of North Carolina.
In addition to size standards, the Steel Founders' Society of America (SFSA) specifications for olivine sand require that the loss on ignition (LOI) must be 1.35% or less. Washington state olivine deposits are so pure that the mined olivine need merely be crushed and sized to meet the SFSA specifications. North Carolina dunite, however, must be beneficiated in addition to crushing and sizing to meet the SFSA LOI specifications. The LOI is derived mostly from the water of hydration of talc, serpentine and similar minerals occurring as accessory minerals in the dunite.
The beneficiation technique commonly used to upgrade North Carolina dunites is gravity tabling. But even with beneficiation, the dunite is selectively mined to insure that the amount of gangue material is not too high. Olivine containing about 3.5% LOI, for example, is now considered too impure to be commercially beneficiated.